Solid state electromagnetic radiation detectors have been developed for consumer, commercial, scientific, medical, military and industrial applications. Consumer applications range from video to high density television. Industrial uses include robotic and machine vision; electronics imaging for advertising and communication; integrated text; and images in office work and publishing. Image sensors are also used for medical (mammography, chest x-rays), astronomy, spectroscopy, surveillance, airport luggage inspection, inspection for foreign objects in foodstuffs, non-destructive testing in industry, and many other applications.
Solid state devices used for detecting electromagnetic radiation, such as x-rays, infrared radiation, ultraviolet radiation, and visible light, store the image momentarily and then, after a selected time interval, convert the image to an electrical signal. A variety of solid state detectors are known. One type of solid state detector is the "hybrid" detector. A hybrid detector generally comprises a pyroelectric material that is bonded to a field effect transistor ("FET"). The FET in such detectors is used as an amplification means to amplify the signal from the detector before the signal is sent to the read-out electronics. Crystalline pyroelectric materials such as strontium barium niobate, lead titanate, and triglycine sulfate ("TGS") are well known in the art. In addition, films of organic polymers such as polyvinylidene fluoride and polyacrylonitrile have also been used as pyroelectric materials.
For example, U.S. Pat. No. 3,809,920 teaches the use of a polyvinylidene fluoride film in conjunction with an FET as being an effective and useful infrared radiation detector.
U.S. Pat. No. 4,024,560 discloses an infrared detector which is a combination of a pyroelectric body secured by electrostatic bonding to the gate area of a field effect transistor such that the pyroelectric body is interposed between the semiconductor body and the gate electrode. In this position, the pyroelectric body forms the gate dielectric of the device. A pyroelectric crystal is typically cleaved, or cut, to form the pyroelectric body.
Japanese Kokai (Laid-Open) Publication JP58-182280 discloses a photodetector comprising a thin film FET and a pyroelectric material. The pyroelectric material forms the gate dielectric layer in this device.
Previously known hybrid structures suffer from a number of drawbacks. One drawback of hybrid structures concerns the pixel size of such devices. Generally, pixel size corresponds to the resolution of a detector. A smaller pixel size means a higher density of pixels for higher resolution. In previously known hybrid structures, the pyroelectric material has been positioned as the gate dielectric layer of the FET. As a result of this approach, achieving smaller pixel sizes has been limited by the size of the pyroelectric material. Because the pyroelectric material of these devices is individually bonded to the field effect transistor, it has been difficult to achieve pixel sizes on the order of 1 mm.times.1 mm or less.
As another drawback, the active detection area of such devices is, at most, only a few square centimeters in size.
As another drawback, hybrid structures tend to be susceptible to harm caused by events such as radiation induced damage. For example, if too much voltage is applied to such detectors, such voltage can irreparably damage the pyroelectric material, i.e., the gate dielectric layer, of the FET. This kind of damage could impair the performance of, or even destroy, the detector.
As a consequence of these drawbacks, previously known hybrid structures have not been practical for high density, large area applications.
Solid state detector arrays have also been known. One type of solid state detector array is the charged coupled device ("CCD"). In essence, a CCD is a shift register formed by a string of closely spaced MOS capacitors. A CCD can store and transfer analog-charge signals, either electrons or holes, that may be introduced electrically or optically.
In Japanese Journal of Applied Physics, vol. 27, no. 12, December 1988, pp. 2404-2408, Hiroshi Tsunami et al. discuss the application of CCD's to take x-ray images of about 8 keV and 1.5 keV for different objects. High resolution CCD sensors which have more than 2 to 4 million pixels have also been reported, for example, in the Proceedings of Electronic Imaging West, Pasadena, California, pp. 210-213 (Feb. 25-28, 1990); and in Electronic, pp. 61-62 (Feb. 29, 1988).
The high cost of the CCD, however, has been a barrier to widespread commercial acceptance of these devices. CCD's, too, require an optical system in order to enlarge the field of view. The use of an optical system, unfortunately, causes a significant reduction in quantum efficiency. This makes it impractical to use the CCD for large area detectors. To date, the largest CCD array reported has been less than one square inch in size.
Amorphous silicon recently has become a material of choice in many solid state detector applications due to its capability for large area deposition and the low cost of amorphous silicon detectors. Amorphous silicon-based solid state detectors generally have been in the form of a linear array. Such devices have gained widespread acceptance for use as monolithic, full page high resolution detectors, due to the following advantages: (1) large area deposition capability, (2) low temperature deposition, (3) high photoconductivity, (4) spectral response in the visible light region and (5) high doping efficiency.
An amorphous silicon linear array is discussed by Toshihisa Hamano et al. (Proc. of the 13th Conference on Solid State Devices, Tokyo, 1981, Japanese Journal of Applied Physics, Vol. 21 (1982) supplement 21-1, pp. 245-249). In this structure, metal (Au, Ni, or Cr, thickness of 3,000 angstroms) is used for the bottom electrode and Indium Tin Oxide transparent conducting film is used for the top electrode. Glass plates (Corning 7059, PYREX) are used for the substrate. Amorphous silicon (a-Si:H) film with a thickness of 1 micron is deposited by plasma-enhanced chemical vapor deposition technique onto the substrate.
For x-ray applications, U.S. Pat. No. 4,675,739 describes a solid state linear array made from photosensing elements. Each photosensing element includes back-to-back diodes: one a photoresponsive diode and the other, a blocking diode. Each of the diodes has an associated capacitance formed by its electrodes. The magnitude of the charge remaining on a given capacitor is sensed and relates back to the intensity of the incident radiation impinging upon the photosensitive diode. In this structure, an amplifying means, i.e., a field effect transistor is not used.
Solid state detectors in the form of a linear array, however, must be moved in order to get a two-dimensional image. This introduces a long read-out time, which makes real-time read-out impractical. This drawback prevents the linear array detector from being used in applications where high speed is required, e.g., medical x-ray applications.
U.S. Pat. No. 4,689,487 describes the use of a large area solid state detector (40 cm .times.40 cm). The solid state detector includes pixels in the form of a 2,000.times.2,000 matrix. Each pixel consists of a photodiode conductively connected in parallel to a capacitor. The photodiode and the capacitor are both then conductively connected to the drain of a metal-oxide-semiconductor field effect transistor (MOSFET). The photodiodes are of a polycrystalline or amorphous material This diode-MOSFET device has at least four main drawbacks. First, a non-destructive read-out cannot be used. Second, the sensitivity of the device is low. Third, the diode has to be operated in the forward mode in order to turn on the transistor. Fourth, the device requires at least 8 complex microlithography and deposition steps for fabrication, causing yields to be low.
U.S. Pat. Nos. 4,606,871, 4,615,848, and 4,820,586 disclose a pyroelectric material that is a blend of polyvinylidene fluoride ("PVF.sub.2 ") and at least one polymer miscible therewith at a temperature above the melting point of the PVF.sub.2. The film may be polarized to render the PVF.sub.2 blend pyroelectric and isotropically piezoelectric. Example 10 in each of these patents describes the coating of an integrated circuit slice of a single crystal silicon chip with the PVF.sub.2 blend, followed by the sputtering of gold onto the surface of the PVF.sub.2 for poling.